أنشئ حسابًا أو سجّل الدخول للانضمام إلى مجتمعك المهني.
Thanks Mr.Omar for invitation,
In fire, concrete performs well – both as an engineered structure, and as a material in its own right.
Because of concrete’s inherent material properties, it can be used to minimise fire risk for the lowest initial cost while requiring the least in terms of ongoing maintenance. In most cases, concrete does not require any additional fire-protection because of its built-in resistance to fire. It is a non-combustible material (i.e. it does not burn), and has a slow rate of heat transfer. Concrete ensures that structural integrity remains, fire compartmentation is not compromised and shielding from heat can be relied upon.
Benefits Concrete as a materialConcrete does not burn – it cannot be set on fire unlike other materials in a building and it does not emit any toxic fumes when affected by fire.
Concrete is proven to have a high degree of fire resistance and, in the majority of applications, can be described as virtually fireproof. This excellent performance is due in the main to concrete’s constituent materials (cement and aggregates) which, when chemically combined within concrete, form a material that is essentially inert and, importantly for fire safety design, has relatively poor thermal conductivity. It is this slow rate of conductivity (heat transfer) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage.
Concrete structures
Thank you my brother for invitation ,,, I agree with the answer of my brother Jaffer and I would lick to thank you again for your question which it very important for all , and in order to get useful answer we must gowing deeply as we can to know some information about the tow type of this structures and its materials and how it will deal with the fire resistance and protection of construction materials Most materials of construction require insulation to achieve resistances that are commensurate with performance requirements in building codes. The amount of protection that designers must provide depends on the inherent resistance of the construction materials, geometry of structural components, function in the structural system, and performance objectives for the building. Fire protection for structural components is accomplished by some form of insulation, usually as applied coatings, encasement of components, and enclosures around components that separates the structural component from the fire environment.Some protection products and systems maintain a barrier to the transmission of heat by their static structure and form (e.g., enclosing steel columns in a fire-rated enclosure). Other protection means dissipate heat energy by physical or chemical transformations. Physical transformations include release of entrapped moisture (e.g., heat of hydration for bound water in spran applied insulation). Chemical transformations include endothermic decomposition and heat-induced expansion to create insulation layers (e.g., intumescent coatings). Examples of these products and system types are discussed in .The selection of materials for critical structural components will have a direct impact on strategies for providing fire resistance. For example, reinforced concrete components with appropriate detailing can sustain the effects of fire temperatures for relatively long periods if they have an adequate concrete cover over the steel reinforcement the concrete cover over the steel reinforcement acts as insulation, and delays temperature rise in the steel reinforcement if it does not spall or crack during the fire event. The dimensions and the thermal conductivity of concrete components may be designed so that substantial time is required for the temperature of the steel reinforcement to rise to damaging levels.
Brick and concrete masonry components are similar in behavior to concrete components: they are relatively large in dimension and tend to insulate any embedded reinforcing steel. Steel structural components subject to fire exposure generally need protective insulating layers such as spray applied insulation, enclosures, or concrete coatings for fire protection. This is particularly true for lightweight steel systems such as cold-formed steel components and fabricated components such as steel joists, which have large surface-area-to-volume ratios.Some building codes place criteria (i.e., allowable building height and floor area) on buildings as a function of occupancy, combustibility of materials of construction, and levels of protection. In those circumstances, the occupancy and size of a building can affect the suitability of certain materials of construction.
FIRE-RESISTANT DESIGN OF CONCRETE STRUCTURES
This section discusses design practices for common types of concrete construction and the response of reinforced concrete structural components under exposure to fire, with an emphasis on concrete floor systems. Refer to Buchanan (2001) for worked examples of concrete component designs for fire exposure.
Concrete Floor Beams
A reinforced concrete beam supporting a concrete floor will have positive moment steel positioned near the bottom of the beam at midspan. The top of the beam usually is integral with the concrete slab and is, therefore, substantially isolated from exposure to high temperatures from a fire below. Since conventional flexural design of beams neglects the concrete below the neutral axis, loss in strength in this portion of the concrete component does not have a significant impact on load-carrying capacity. The integrity of the steel reinforcement in this region, on the other hand, is critical. The concrete cover over the steel reinforcement acts as an insulator. Thus, when a concrete beam is exposed to fire, the temperature increases rapidly at the section surfaces but more slowly at the interior. The temperature profile across the section drops substantially within a short distance from the exposed surface, resulting in a highly nonlinear temperature profile. Since concrete has a relatively large heat capacity and density, there can be a significant time lag before the temperature of the steel reinforcement increases. Hence, the temperature of the steel reinforcement lags the concrete surface temperature.
During initial phases of a serious fire, the concrete cover normally will remain intact,providing insulation to the steel reinforcement. However, high temperatures weaken concrete, produce high stresses due to thermal expansion, and generate vapor pressures within the concrete which may cause spalling
If the concrete cover over the reinforcing steel spalls during the progression of a fire, the steel is exposed directly to the fire environment. Without insulation, the steel heats rapidly, with a
corresponding reduction in strength and stiffness. Even if the concrete does not spall, the temperature of the steel reinforcement may rise if there is sufficient fire duration (ACI216.11997).
Untested Concrete Component Capacity in Design Fires
When a specific design is not addressed by standard fire test results, and when informed judgment will not allow designers to adapt test results, designers need to conduct analyses to validate fire resistance designs. These analyses can follow two paths:
1. Rely on published charts that show the theoretical temperature at depth into concrete components as a function of duration in standard exposure fires ).
2. Conduct heat transfer analyses to determine internal temperature increases as a function of time.
Standard fire exposure charts can be adapted to show approximate theoretical temperatures inside concrete components during realistic fires. Some rules for determining direct relations are available and designers can rely on fire conversion formulas that give equivalent standard fire exposure times for realistic fires when fire load, ventilation, and compartment surface materials are known. Heat transfer analyses are useful when standard exposure curves cannot be used and when conversions from realistic fire exposures to standard
fire exposures are impractical or insufficiently accurate.With either approach, the goal is to determine the temperature of the steel reinforcement due to the fire exposure, and to find the conditions and durations at which the load carrying capacity of the concrete component has been reduced so that it no longer supports the design service load.
The design service load for analysis of fire effects is usually taken as the “point-in-time” load
rather than the full load when such loads control for non-fire conditions, which may be taken as
Restraint of Continuous Concrete Floors
Continuous concrete floors may not fail when the positive moment flexural steel at midspan has inadequate strength for the design service loads. Plastic hinges need to form at the support points in addition to the midspan before a failure mechanism is created. In concrete structures with continuous beams, the steel reinforcement for negative moment resistance is embedded in the slab near the top of the beam, and is likely to be relatively protected from the heat in the fire compartment. Hence, formation of negative moment hinges due to reduction in steel reinforcement capacity usually trails the formation of positive moment hinges.
Thermal Expansion Effects
Fully developed fires in a compartment generate intense heat in the compartment. However, adjacent compartments are usually not significantly heated. As the fire spreads in a building, new areas sequentially become hot as the fire intensifies locally while an area of previously intense fire begins to cool. In a fire environment, forces generated by restraining thermal expansion may have additional effects on component and system survivability. Differences in temperatures of structural components within a structural system may create restraining forces that affect the load-carrying capacity in structural components. For instance, when a fire first begins to heat structural components, those components will expand and push against adjacent structure. If there is continuity within the structural system and adequate strength in the surrounding structural
components to resist forces due to thermal expansion, the heated components may develop significant compressive forces. In reinforced concrete floor beams, thermally induced compression can initially counteract the effects of reduced strength in steel reinforcement due to heating. As the steel tensile strength is being reduced, thermal expansion effects are generating compressive forces and reducing the demand on the steel reinforcement, prolonging the load-carrying life of the beam. However,
thermally induced compressive forces along the beam length may also generate moments at the center of the beam, which would increase demand on the steel reinforcement.
Reinforced Concrete Floor Sagging
As steel reinforcement temperatures increase, steel stiffness and strength will reduce and floor beams will begin to sag. This deformation could signal failure to contain the fire in the compartment if the floor sagging is sufficient to cause a breach in the fire barrier (e.g., a gap or opening between the floor and the walls). However, large deformations do not necessarily mean that beams have failed to perform adequately in a fire, particularly if the performance goal is prevention of collapse.
In structural systems with adequate continuity, large deformations and a significant loss of stiffness can generate tensile forces in the floor beams. Once deflections become large, the weakened reinforcing steel usually has adequate strength to support the applied loads with the beam acting primarily as a tension member. For this mechanism to develop, splices in the reinforcing steel need to function, and there must be sufficient continuity and strength in the beam detailing and in the structural system around the bay with large deformations to support the tensile reactions generated at the ends of the beam.
Cooling Phase Effects Assuming that heated beams do not collapse during the fully developed phase of a fire, they may develop significant additional forces during the cooling phase. Surviving components that previously expanded and sagged under the influence of heat will begin to cool and contract. This contraction pulls inward on the adjacent structure, particularly if components were deformed during the fire event. Floor beams with thermally induced compressive loads that were sufficient to cause plastic strains to develop may experience tension as contraction during cooling reduces,and possible reverses, the compressive forces on adjacent components. During a fire event, the sequence of reinforcing steel yielding, concrete weakening and spalling, midspan and end supports undergoing large rotations, floor beams developing compressive and tensile forces may cause stresses that exceed the level normally expected in concrete component, particularly for steel reinforcement details at connections.
Design of Reinforced Concrete Components
Conventional design of reinforced concrete components usually does not consider the load carrying capacity after flexural failure. For unrestrained, determinate components, failure usually is assumed when the first plastic hinge forms. For continuous beams, approaches for determining the benefits gained by compression while restraining component expansion (when connections and the surrounding structure can sustain the forces associated with restraint of expansion) and by redistribution of moments that occurs as hinges form are presented . When analyses show that the strength of reinforced concrete cannot provide adequate resistance to the effects of fire, within the limitations of other design constraints, the designers have two principal options: change the configuration of the components or apply methods to modify the design-basis fire.
Configuration changes can take the following forms:
Add restraint.
Add continuity.
Increase concrete cover over reinforcing steel.
Increase the area of reinforcing steel.
Each of these options adds resistance at a cost. The designer needs to anticipate cost, together with other goals of the design, when selecting the best method or combination of methods. Normally, the addition of restraint and continuity to systems is costly and potentially disruptive to the intended behavior of structural systems and may affect space usage. Enhancement of concrete cover can usually be accomplished at a modest cost, with the benefit of additional insulation provided for the steel reinforcement. Introduction of additional reinforcing steel (beyond that required for design loads under ambient conditions) to provide reserve moment capacity also can be accomplished at a modest cost. Steel reinforcement can often be increased
in diameter or added without changing the profile of reinforced concrete components. When changes in the design of components does not emerge as the preferred approach to adding fire resistance, the performance of reinforced concrete components can be enhanced with many of the same fire protection methods that are available for structural components of other construction materials: concrete components can be insulated from the harsh fire temperatures. To evaluate these methods, the designer will need to refer to qualifying fire tests or pursue heat transfer and structural response analyses to evaluate the effectiveness of various protection methods.
FIRE-RESISTANT DESIGN OF STEEL STRUCTURES
This section discusses design practices for common types of steel construction and the response of steel structural components and composite floor systems under exposure to fire. Refer to Buchanan (2001) for worked examples of steel component designs for fire exposure. Steel Behavior at Elevated Temperatures, unprotected steel components are sensitive to the effects of fire. The relatively high thermal conductivity of steel as compared to approximately W/m2K for normal weight concrete) and generally thin proportions of steel components make unprotected steel structures susceptible to rapid heating when exposed to fires. For this reason,steel components in structures usually require passive fire protection. When structural steel component temperatures exceed400 °C, the yield strength and modulus of elasticity begin to decrease . As temperatures increase in a steel section, its yield strength and elastic modulus decrease. When the yield strength is reduced to the applied stress level, the steel section will begin to yield (i.e., deform under plastic strains and, possibly, creep strains). At the same time, reduced stiffness will increase deflections.Composite Floor Behavior at Elevated Temperatures
Heating of a composite floor initially causes thermal expansion, which in turn, subjects the floor section and adjacent framing to compressive loads. If a steel beam acts compositely with a concrete slab, the most highly stressed element of the steel beam is typically the bottom flange. If the bottom flange yields as steel temperatures increase, the neutral axis shifts upward. Since thin webs make only minor contributions to moment resistance, yielding of the bottom flange can result in rapid and significant loss of moment capacity as the cross section yielding progresses rapidly upward through the web. If a composite section is heated sufficiently, large deformations and sagging may occur. The temperature gradients in the steel beam and slab will induce thermal bowing, where the floor section will bow downward to relieve the differential thermal expansion. The connections and surrounding framing will be subject to compressive loads from thermal expansion.Consider a section of the composite floor that includes a beam and tributary area of the slab, with
a protected beam (i.e., insulated with passive fire protection). If the floor section is heated from fires below, typically the bottom flange will have the highest temperatures, as it heats first, with a temperature gradient through the web to the top flange. The top flange will be considerably cooler (often by several hundred degrees), due to contact with the thermal mass of the slab, which acts as a heat sink. If the bottom flange temperatures reduce the yield strength to equal the applied loads, the composite section will develop plastic strains at the point of highest load, which is often near the midspan of simply supported floors. If the plastic strains result in a plastic hinge forming at the highly loaded section, the floor may ‘hang’ between supports. If the sagging increases to the point that the composite floor is supporting its loads through tensile loads at its connections, then the floor section is often described as being supported through catenary action. The top flange and steel reinforcement of the slab will attempt to carry the tensile stresses, if the remaining capacity is greater than the supported loads. Six fire tests of an8-story steel framed structure with composite floors was carried out by ;
research team at the Building Research Establishment (BRE) in Cardington, Bedfordshire to: (1)
gain understanding of the natural fire resistance of such structures, (2) correlate data and observations with predictive numerical models, and (3) establish a more rational design methodology for steel framed building response to fire (British Steel1999). There are a number of reports and papers written that summarize the test data, observations, and numerical analysis of the tests. British Steel (1999) and O’Connor (2003) are given as summary papers.
Design of Composite Floor Section
The design of steel components for fire exposure is usually based on the assumption that positive and negative moments are limited to the plastic moment, which is based on a reduced yield strength and stiffness based on the maximum temperature estimates. The flexural capacity of a composite floor section is determined by calculation of the plastic moment across the steel beam and slab section for the estimated maximum temperature that the steel beam components will reach due to the design fires. When determining the flexural capacity for a thermal condition, one should also consider the possible reduction in the stiffness and strength of the concrete and whether the concrete can support the compressive load(meaning that a hinge has not formed and the beam still has some flexural capacity) or if the concrete will crush (signaling the limit of flexural capacity).
Steel beams connections can be designed to carry increased tensile loads for the condition where
hinges develop in a composite floor section and cause tensile loads at the supports. Large deformations associated with loss of stiffness and flexural yielding can allow the full cross section of steel beams to act as tensile elements as long as the connections can sustain the substantial deformations associated with large hinge rotations and the surrounding structure can support the forces induced by tension in the beam. It is not common to design for loss of flexural capacity. However, conventional design does allow full redistribution of moment between the positive and negative moment regions to account for the formation of full hinges at these locations. For steel components to develop full plastic hinges they must be adequately braced at hinge locations to undergo the associated rotations. In addition, the designer must consider the integrity of the bracing element; it must also be designed with consideration for the effects of temperature on its strength and stiffness.
Passive Fire Protection for Steel Components
Common methods to protect steel components from the effects of fire include spray-on fire resistive materials (SFRM), intumescent coatings, and enclosures of gypsum board, mineral fiberboard, concrete, masonry, or similar materials each of these protection systems provides its insulating function through one or more mechanisms: low thermal conductivity, high heat capacity, heat-absorbing reactions, or formation of insulation layers through expansion. Each protection system has its set of advantages regarding cost, aesthetics, weight, and ease of installation. The systems with perhaps the longest history of use generally are relatively easy to install by trade personnel. Systems, such as enclosures with gypsum or mineral fiberboard products, are easy to install and relatively lightweight. Non-combustible enclosures of masonry materials are also relatively easy to install on columns (but not beams), but they have a weight premium that should be considered in design. SFRM coatings are lightweight, but generally require an enclosure to conceal their unfinished appearance unless they are applied in areas where finishes need not have high aesthetic qualities. SFRM products are somewhat vulnerable to damage over time, particularly in areas where work may be done by other trades. An inspection and maintenance program by the owner can address any loss in integrity of the SFRM coating.Some of the less-traditional approaches to fire protection of steel, such as intumescent coatings, are still costly when compared to other approaches and often need to be applied by specialty
contractors. Some intumescent coatings can be applied as the final finish for exposed steel components, thereby adding to the steel framing appearance for architectural purposes.
